Piezoresistive heat engine and refrigerator

Heat engines provide most of our mechanical power and are essential for transportation on the macroscopic scale. However, although significant progress has been made in the miniaturization of electrostatic engines, it has proved difficult to reduce the size of liquid- or gas-driven heat engines below 10 7 μm 3 . Here we demonstrate that a crystalline silicon structure operates as a cyclic piezoresistive heat engine when it is driven by a sufficiently high d.c. current. A 0.34 μm 3 engine beam draws heat from the d.c. current using the piezoresistive effect and converts it into mechanical work by expansion and contraction at different temperatures. This mechanical power drives a silicon resonator of 1.1×10 3 μm 3 into sustained oscillation. Even below the oscillation threshold the engine beam continues to amplify the resonator’s Brownian motion. When its thermodynamic cycle is inverted, the structure is shown to reduce these thermal fluctuations, therefore operating as a refrigerator. A micrometre-scale device that exploits the piezoresistive characteristics of silicon acts like an engine, converting heat into mechanical work in one mode of operation, and, in another, like a refrigerator, suppressing mechanical fluctuations.